Joint replacement surgery, also known as total joint arthroplasty, is performed on individuals who have a joint that because of arthritis or other condition or injury requires a replacement or reshaping of the joint surfaces. Typical knee replacement surgery involves the shaping of the distal femoral bone using a special cutting jig that has been placed on the end of the femur. The cutting jig is aligned with the mechanical axis of the leg so that the replacement knee is properly aligned, even if the patient was originally knock-kneed or bowlegged. The distal end of the femur is cut and shaped to accept an implant surface to function as part of the replacement knee. In addition, the tibia is prepared in a similar manner using another special cutting jug to assist the surgeon in making a resection of the tibia so that the implant will be properly aligned in the new knee. Also, the patella is prepared by typically removing a portion of the undersurface of the patella. In a similar manner, hip replacement surgery involves the preparation of the acetabular cup to receive a replacement surface and the femoral head is replaced with a stem and ball to match the cup implanted into the acetabulum. For the shoulder, the glenoid is not usually replaced but the surface is modified to accept an implant placed in the end of the humerus bone.
After the bones surfaces and/or the tissue has been prepared, the implants are then placed into position on the prepared surfaces. For instance, in knee replacement surgery, in addition to implants on the surfaces of the prepared femur and tibia, the replacement knee joint also typically will include a spacer to mimic the effect of the knee cartilage. Often a surgeon will first insert temporary or trial implants into position within the prepared joint and then manipulate the joint to be sure the implants will function properly and the joint will be stable and have a sufficient range of motion.
Orthopedic surgeons have been using surgical navigation systems for some time to assist in properly locating and positioning the cutting jigs used to make the resections of the bone to prepare the joints to accept the replacement implants. However, in the past the use of a surgical navigation system required the surgeon to insert markers that could be seen by the pre-operative scans and were visible to the surgeon so that the surgeon could register the pre-operative scan to the patient's leg and knee joint during surgery. This has involved either semi-permanent markers or fiducials placed into the patient's tissue before the preoperative scans are made and requiring that the patient maintain the fiducials in place until surgery, or temporary fiducials that are removed after the pre-operative scan is done and replace just prior to surgery. Each of these systems has disadvantages. While the use of the semi-permanent fiducials insures proper registration of the scan to the patient's leg and joint, there can be significant discomfort in the insertion and maintenance of the fiducials in place between the time of the scan and the date of surgery. The use of the temporary fiducials eliminates this problem, but these temporary fiducials are not as reliable since they must be replaced in exactly the same positions for proper registration to occur. In each of these surgeries and in other related surgeries, it is now possible to perform these surgeries using digitized landmarks that are determined intra-operatively without the need to have a pre-operative scan. However, it is desirable to incorporate images into the display screens used by the surgeons to locate the jigs and other cutting tools so that the proper resections of the bone are made to receive the implants to repair the joint. In this instance, the image is used as a background guide and the image is not used as the basis for accurately positioning the tools and jigs.